CAR is different than a conventional phased array radar. Sensitivity is often limited in CAR compared to a conventional phased array radar because energy is typically collected in a relatively short period of time, consisting of a single radar range/Doppler acquisition when using CAR. Because of this the total received energy is lower than, say, a conventional phased array radar that forms a directive beam but then collects energy over a longer period of time by sequentially moving the beam to all beam directions within the field of view. For example, if N beams fill the field of view, then a conventional phased array radar requires one acquisition period at each beam position, so the total energy will be N times greater than for CAR. Two type of CAR (Type I and Type II) are identified in U.S. Provisional Patent Application Ser. No. 61/912,990, filed Dec. 6, 2013 and entitled “A Method and Apparatus for Processing Coded Aperture Radar Signals” and U.S. patent application Ser. No. 14/561,111 filed on the same date as this application and entitled “Method and Apparatus for Processing Coded Aperture (CAR) Radar Signals”. This disclosure described in greater detail Type II CAR and in particular how it reduces ambiguity (from multiplicative noise) compared to Type I CAR.
If one extends the Type I CAR acquisition period a factor of N then the sensitivity of CAR will be the same as for a conventional phased array (other things being equal). However, in that case the number of Doppler bins increases a factor of N, increasing the computational overhead of the Type I CAR system.
Given the typical lower sensitivity of a Type I CAR coded radar system, it can effectively be employed where a short range radar can be used, for example, as an automotive radar, for rotorcraft landing in degraded visual environments, proximity sensors, aircraft altimeters (used using landing), for aircraft maneuvering on a taxiway or on an aircraft carrier, etc. However, as noted above, the sensitivity of a Type I CAR coded radar system can be increased if needed.
Type I CAR provides a method and apparatus for acquiring information about the 3D location and radial velocity of a continuum of scatterers within a relatively short acquisition period compared to conventional radar as mentioned above. The Type I CAR technique has been previously made the subject of patent applications (see the US Patent Applications identified above), along with suggestions for coding the single bit phase shifters located at each aperture transmitting and/or receiving element. However, the previously disclosed techniques produce a uniformly distributed ambiguity, sometimes called residual ambiguity or multiplicative noise, which reduces the sensitivity and dynamic range of the radar.
Type II CAR described herein addresses this residual ambiguity or multiplicative noise associated with Type I CAR.